This invention relates generally to a video display apparatus, and more particularly to a back-light device for illuminating a transmission-type video display plate within the video display apparatus.
A conventional video display apparatus which employs a video display plate such as a transmission-type liquid crystal panel requires that a light source be positioned behind the panel, that is, as a back-light device. Generally, a fluorescent lamp is used as the back-light device. The fluorescent lamp provides a high level of luminance, a high color temperature, a high level of effectivity and is relatively inexpensive. Unfortunately, a significant decrease in the level of luminance produced by the fluorescent lamp occurs at extremely low temperatures and especially at temperatures below the freezing point.
As shown in FIG. 2, the luminance characteristics of a cold cathode fluorescent lamp serving as a back-light device in a conventional video display apparatus varies based on ambient temperature conditions. More particularly, the level of luminance at low temperatures markedly decreases. At about -20.degree. C., the level of luminance of a cold cathode fluorescent lamp is about one fourth the level of luminance produced by the cold cathode fluorescent lamp at normal ambient temperatures of about 25.degree. C.-30.degree. C.
To compensate for low levels of luminance at low ambient temperatures, the level of electric power provided to the back-light device is varied based on the ambient temperature. For example, as shown in FIG. 3, a back-light device 50 includes a battery 14 which supplies a voltage to a voltage regulator 15 which is further modified by a variable resistor 16. Based on ambient temperature conditions, variable resistor 16 is adjusted to provide a suitable voltage to operating circuit 17 which in turn provides a suitable voltage to fluorescent lamp 18 for the latter to produce an acceptable level of luminance.
Another method for adjusting the level of luminance of a fluorescent lamp is disclosed in Japanese Patent Laid-Open Application No. 58-80299. The Japanese application discloses a method for varying the oscillating frequency of a high frequency operating circuit by changing the value of a variable resistor based on the ambient temperature. Current flowing through the fluorescent lamp at low temperatures is increased to compensate for an otherwise low level of luminance.
Video display apparatuses for indoor use are not limited to any particular type of power source and can employ a variety of power sources for supplying power to their components (including direct connections to an electrical wall outlet). Power losses associated with these components is not a significant problem. Video display apparatuses for outdoor which include, for example, pocket size televisions and electronic viewfinders of video cameras are powered by a battery. Significant power losses associated with such video display apparatuses are undesirable and a serious drawback.
The luminance compensation technique illustrated in FIG. 3 requires a division of the voltage between voltage regulator 15 and the rest of the back-light device circuitry. In particular, a relatively large voltage drop is developed across voltage regulator 15. Power consumption increases.
The luminance compensation technique disclosed in the Japanese application requires the impedance of impedance elements which are part of the load be changed to increase or decrease the current flowing through the fluorescent lamp. Furthermore, although the method disclosed in the Japanese application consumes less power than circuit 50 of FIG. 3, the variable resistor required to change the oscillation frequency of the high frequency operating circuit must be adjusted manually (by hand) based on a temperature detector which detects the ambient temperature conditions. The range (width) of oscillating frequencies also must be increased (widened) to provide an adequate flow of current through the fluorescent lamp at low temperatures. An independent oscillation circuit is required. It is also difficult to increase the range of oscillating frequencies when using self-excited oscillation circuits which include transistors.
Accordingly, it is desirable to provide a back-light device for use with a video display apparatus which is not adversely influenced by low ambient temperature conditions. In particular, it is desirable to provide a back-light device which automatically maintains the level of luminance produced by a fluorescent lamp at a suitable level for video display purposes at low ambient temperatures.